A light-emitting diode (LED) emits incoherent narrow-spectrum light when the diode is electrically biased in the forward direction of the p-n junction inside of the diode. LEDs typically have higher luminous efficacy, i.e. lumens per watt, than conventional light sources such as incandescent bulbs. LEDs are often used in battery powered or energy saving devices, and are becoming increasingly popular in higher power applications such as, for example, flashlights, area lighting, and regular household light sources.
Due to the emitting spectrum nature of the LEDs, the quality of the light is a primary consideration with the use of LEDs in higher-power applications. It is desirable to have high brightness white LED device that has better light quality, which is quantitatively measured by the color rendering index (CRI). The CRI is a measure of how true the light is as compared to an ideal or natural light source in representing the entire light spectrum. An ideal or natural light source has a high CRI of, for example, 100. Individual white LED typically has a poor CRI, in the approximate range of 70-80, because of their emitting spectral concentration. To partially remedy the problem, phosphors are utilized to convert the wavelength of the light emitted from the diode to other wavelength regions. Furthermore, LEDs with different emitting colors mixed to produce a white light better filling out the light spectrum. For example, combinations of white, amber, red, and green LEDs can provide light with CRIs at or above 90.
Combinations of LEDs having different emitting light colors may include multiple strings of LEDs having the same emitting light color. There are conventional approaches for modulating the light output from each string of LEDs having the same emitting light color. One approach is to provide a constant current source and turn the string of LEDs on and off over a particular duty cycle to change the perceived light intensity of that string. This is achieved by using switch-mode transistors switching on and off at a high frequency. The approaches are used not only to change the relative intensity of LEDs with different colors but also to raise and lower the overall intensity of the string in a manner similar to a dimming function. Although the approach provides the color control, it has significant efficiency penalties.
This approach uses a current source for each LED string and modulates the duty cycle of the LED string at a frequency imperceptible to the human eye. But, running the LEDs at their full current rating and duty cycling their outputs is typically less efficient than simply running the LEDs continuously at a lower current, because LED efficiency declines with increasing current.
Furthermore, the switching circuit introduces electromagnetic interference (EMI), whose disadvantage cannot be understated. To filter and screen the EMI, more components need to be taken into account, driving up the parts cost.
Moreover, the incandescence emission from a conventional incandescent light bulb is a black body radiation. Its emission spectrum conforms to the Planckian locus on the CIE color space. When dimming an incandescent light bulb, customers are used to the color temperature changing according to the black body radiation. Therefore, it is desirable to mimic the color temperature change conforming to the Planckian locus when dimming a light source. However, an LED barely shifts its emission spectrum when the pass-through current reduces. Therefore, when dimming a LED-based lighting fixture, a user does not observe the color shift as he is used to.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings.